Engineers typically design high-pressure oil field pumps in two sections; the (proximal) power section (herein “power end”) and the (distal) fluid section (herein “fluid end”). The power end usually comprises a crankshaft, reduction gears, bearings, connecting rods, crossheads, crosshead extension rods, etc. Also located in the power end of a mud pump is at least one liner within which a piston is moved in a reciprocating manner by a piston rod. Each liner comprises a cylindrical sleeve within a steel hull. Notwithstanding their location in the power end frame liners, pistons and piston rods are considered part of a mud pump's fluid end.
Commonly used mud pump fluid ends also typically comprise a suction valve and a discharge valve associated with each liner (together with its piston and piston rod) in a sub-assembly, plus retainers and high-pressure seals, etc.
FIG. 1 schematically illustrates a cross-sectional view of a typical mud pump fluid end, showing its connection to a power end frame. A plurality of sub-assemblies similar to that illustrated in FIG. 1 may be combined in a mud pump.
FIG. 2 schematically shows a cross-section of a typical mud pump liner together with its piston and piston rod. High-pressure pump piston designs for mud pumps have evolved over several decades, as indicated in U.S. Pat. Nos. 2,473,064; 4,270,440; 4,516,785; 4,601,235; 4,735,129; and 5,480,163, each patent incorporated herein by reference. The designs illustrated in these patents cover a period of more than 50 years. Each incorporates one or more structural features for reducing or preventing extrusion under high pressure of a portion of the piston's elastomeric seal material (e.g., rubber, polyurethane or analogous resilient material) into a space between the piston and the liner wall (the “extrusion gap”). The gap, as shown schematically in FIG. 2, typically arises because the outer diameter of the piston's steel hub is slightly smaller than the liner's inner diameter to permit reciprocating motion of the piston within the liner. As the liner wears the extrusion gap widens, increasing the tendency for sealing material to extrude into the gap under pressure (i.e., during a pump's pressure stroke). During extrusion, the sealing material is damaged or destroyed and the seal begins to fail. Eventually, failure of the seal leads to excessive leakage past the piston, followed by premature failure of the piston and/or the liner.
The tendency of piston seal material to extrude into the gap under pressure is aggravated by the large amounts of frictional heat generated by movement of a tight-fitting piston seal on the liner wall. Unless this heat is effectively removed, it tends to quickly degrade the seal and allow extrusion of seal material into the gap (described as a flow of elastomeric material under pressure in the '163 patent). As pieces of the resilient sealing material flowing into the gap are torn, cut and/or bitten off, excessive leakage develops between piston and liner. Continued seal degradation may allow a piston flange to contact the liner wall and damage it. The patents cited herein describe various inventions to slow seal degradation by reducing the tendency of elastomeric seal material to flow into the gap and/or by eliminating the gap altogether through use of a structure that extends from the piston to the liner wall.
One long-used method of reducing elastomeric flow into the extrusion gap is by molding and/or bonding an elastomeric seal around a strong metal rib or flange that extends radially close to the gap (see, e.g., col. 2 of the '064 patent and col. 3 of the '163 patent). Adherence of the seal material to the metal near the gap is the extrusion control device because seal material bound to the metal can not flow. When elastomer adherence to the metal fails, seal material flows into the gap (i.e., seal extrusion) causing the seal to fail (i.e., to allow excessive leakage of the pumped fluid past the piston). Seal failure also allows the piston's metal rib or flange to contact the liner wall, often leading to galling and rapidly increasing liner wear.
Elastomeric flow through the extrusion gap may also be reduced when a portion of the elastomeric seal near the gap is reinforced by, for example, fabric (see, e.g., col. 4 of the '235 patent and col. 3 of the '129 patent). Reinforcement may also be provided through use of elastomers of different hardness in the piston seal (e.g., dual-durometer urethane seals), with the harder elastomer nearest the extrusion gap. But dual-durometer seals are difficult and expensive to produce with uniformly predictable characteristics. In particular, failure of the intra-seal bond between the harder and softer elastomers may lead to failure of the entire seal. Single-durometer seals avoid the problems of establishing and maintaining intra-seal bonds, so they are less expensive to produce. And single-durometer seals perform adequately except in high-heat and/or high-pressure applications (i.e., except in areas near the extrusion gap), where elevated temperatures rapidly degrade them.
In another of the piston seal embodiments cited above, a relatively rigid split ring (e.g., made from a metal such as steel or cast iron) is bonded to an elastomeric seal that is intended to control expansion of the ring to ideally just “kiss” the liner, thus closing the extrusion-gap (see, e.g., col. 5 of the '785 patent). Unfortunately, since a split ring will not radially expand uniformly around its entire circumference, ideal (i.e., complete) closure of the extrusion gap can not be obtained by practicing the invention of the '785 patent.
An alternative approach to blocking or reducing elastomeric flow is described in the '163 patent, wherein an annular flange combined with an axially-extending annular skirt forms a relatively rigid reinforcement section that prevents elastomeric material radially inward of the skirt from bulging outwardly towards the circumference of the piston (see, e.g., cols. 3 and 4 of the '163 patent). Elastomeric material radially outward of the skirt, on the other hand, is still subject to the elastomeric flow phenomenon noted above.
Yet another approach to high-pressure piston design features use of an annular gap filler ring with controlled radial creep characteristics which urge the ring into continuous contact with the liner such that an extrusion gap does not occur (see, e.g., col. 5 of the '440 patent). Glass-filled nylon is described as a material for the gap filler ring having the desired controlled radial creep characteristics (see, e.g., col. 6 of the '440 patent). But the continuous contact of the gap filler ring with the liner tends to quickly wear the liner's inner surface in a barrel shape (that is, having smaller diameters at the ends than in the middle). As liner wear continues, extraction of the piston through either end of the liner (with the gap filler ring remaining in constant liner wall contact) becomes increasingly difficult and may eventually become impossible.
The '440 patent also describes problems related to frictional heat due to the tremendous force of the piston seal on the liner wall. Dissipation of this heat is handled in the invention of the '440 patent by a plurality of water channels communicating a source of fluid from a passageway within the piston body to the liner for washing and cooling (see, e.g., cols. 1, 2 and 7 of the '440 patent). It is also common practice merely to direct a stream of water at the back of the piston for combined cooling and washing of the piston and liner wall.
The problems associated with frictional heat build-up between high-pressure pistons and liners are exacerbated by higher operating pressures and also by newer liners comprising ceramic and zirconium. Both ceramic and zirconium offer excellent corrosion resistance and a 300–400% increase in wear life over traditional hardened steel liners. But both materials are very expensive and very brittle, and they have the additional disadvantage of having lower thermal conductivity than steel. Their lower thermal conductivity means that they tend to retain the substantial frictional heat that develops when a piston with its tight-fitting seal reciprocates within a liner. This retained heat results in increased piston operating temperatures. And with prolonged exposure to retained heat, elastomeric piston seal materials (particularly urethanes) are progressively degraded. Subsequent seal failures eventually allow pistons to damage the liners in which they reciprocate, as noted above. The likelihood of such damage is relatively lower with use of liners having better thermal conductivity because the liner tends to more effectively remove heat from the piston seal-liner interface.
Compounding the problems with frictional heat retention in pistons and liners is the fact that the designs of currently available piston seals evolved during a time when typical mud pump working pressures were about 2,000 to 4,000 pounds per square inch (psi). Modern mud pumps, in contrast, may operate at pressures two to three times as high. An advanced high-pressure piston is needed that combines better scavenging of frictional heat with reduced extrusion of elastomeric seal material, simultaneously improving seal performance and reducing liner wear to extend the pump's service life.